In large forestry plantations in, for example, South America and Australia, forestry stands grow in which the trees demonstrate a relatively uniform size and with equivalent properties such as density and moisture content. The trees grow rapidly and are planted in rows such that they grow with equal spaces at a separation from each other and on relatively level ground. These conditions make efficient forestry possible in which one or several forestry machines can work along the outer edge of the stand and process the trees at a rapid pace.
The processing often takes place with the aid of accumulating working heads arranged at an arm of a tool carrier in the form of a forestry machine, excavator or similar that is driven by wheels or tracks. The head is attached to the arm by a tool fixture of known type in a manner that allows it to be removed. The term “accumulating working head” is here used to denote a timber working head that is manoeuvred by the arm of the tool carrier to a first tree, the tree is clamped against it by one or several holder arms, and cut by a rotating toothed blade or disc. The working head is subsequently moved to the next tree, which also is clamped against the working head and cut. This continues until the working head is full. When the working head is full, the arm is pivoted away from the working area and deposits the collected bundle of cut trees, after which the procedure is repeated.
The blade or disc of the working head comprises a flywheel with a large mass and with teeth arranged along the periphery of the disc. The disc is accelerated by a motor up to a pre-determined speed before the cutting, after which the cutting is started. It is indicated that the correct speed has been reached by a gauge of the rate of revolution or by a pressure gauge arranged in connection with the disc or the motor. This gauge of the rate of revolution is used also to indicate when the speed of rotation is too low for it to be possible to carry out efficient cutting. As a consequence of the uniform properties and sizes of the trees, it is possible to cut a tree in a very short time, approximately one second or less. When a number of trees have been cut, a waiting period occurs in order for the disc to be able to reach again the correct speed.
The disadvantage of the prior art technology is that when the teeth of the disc loose their sharpness and become blunt after a number of cuttings, the consumption of time for each cutting becomes longer. This means that the working head cannot be filled with trees before the disc must be accelerated up to the correct speed again, whereby the efficiency becomes lower.
Monitoring of the sharpness of the teeth is currently carried out by the driver, who sits in the forestry machine. Not only does production of heat take place when the teeth are blunt and must wear their way through the trunk, but also the driver discovers that the speed of the disc becomes too low before the working head is filled.